Both gel (U.S. Pat. No. 5,079,274) and lightly crosslinked macroporous (U.S. Pat. No. 5,288,307) aromatic polymer adsorbents, which have been post-crosslinked by the methylene-bridging of chloromethylated aromatic copolymers, have been used to adsorb and recover hydrocarbon fuel vapor emissions from fuel reservoirs and volatile organic compounds from gas streams. While the adsorption capacities for methylene-bridged aromatic polymer adsorbents typically surpass those of non-bridged adsorbents and activated carbon, they also suffer from one serious drawback, i.e., they have a tendency to competitively adsorb water at high relative humidities. Not only does this tendency to adsorb water reduce overall capacity for the volatile organic compound, but upon desorption the compound recovered is contaminated with water.
Chloromethylated aromatic copolymer is methylene-bridged by first swelling the resin and then contacting the resin with a Friedel-Crafts catalyst at a temperature sufficient to effect alkylation. Normally, a certain percentage of chloromethyl groups is prevented from reacting with a neighboring aromatic ring because of increasing rigidity of the polymer backbone due to methylene-bridging and because of resulting geometric constraints on any further bridging. Typically, about four percent of the chloromethyl groups remain unreacted. During additional processing or during intended use, the residual chloromethyl groups are hydrolyzed or oxidized to from hydrophilic sites, e.g., benzyl alcohol groups or benzoic acid groups. These hydrophilic sites favor the adsorption of water which can effectively compete with the adsorption of the volatile organic compound and can reduce the effective capacity of the adsorbent. Thus, it would be desirable to prevent the residual chloromethyl groups from being converted into hydrophilic sites. Ideally, the weight of water adsorbed at high relative humidity, i.e., humidities greater than 75 percent, should be less than about 10 weight percent based on the dry weight of the polymer adsorbent.